How big of a space do you have? We did an 1800 sq ft patio for a ca. 1893 farmhouse, and the septic cap was right in the middle of where we wanted the patio. We put an extender on the cap (originally you had to dig down 2 feet to get to the lid) and then left an open 8'x8' square in the patio, centered on the tank. Brought in soil, dry set 18x18 flagstones, and put groundcover in between. Sure, every 2-3 years when it needs pumped the gardener (who they pay me to consult with) will have to pull up and replant the ophiopogon, but it works. You just need to make sure you're not then creating a water trap in the middle of your patio.